I have always been of the opinion that wanting to play an instrument left handed was phsycological because as was said earlier both hands are used, albeit differently.This does not mean that people who insist they can't play right handed are wrong...in order to play well you need to be comfortable.
The problem is that a left handed violin means reversing the whole design and that means that most makers would be effectively working 'left handed' and could therefore make a poorer instrument due to their unfamiliarity with that way of working.